UHP-base alloys have been used to study the effects of sulfur and yttrium on the oxidation resistance of ferritic Fe20Cr5Al alloys. Doping with sulfur and/or yttrium gave a series of alloys with sulfur contents between 0.2 to 50 ppm sulfur, with or without yttrium (0.1 wt% maximum). The a-alumina scale and the metal±oxide interface structures were investigated after oxidation between 1060 and 1350 C (1 to 100 h), using several techniques: SEM, TEM, STEM and Scanning AES. It was found that sulfur contents lower than 1 ppm or doping with yttrium gave flat and adherent scales. The equiaxed structure observed on Y-undoped specimens was transformed into a columnar structure when yttrium was present in the alloy. Sulfur and carbon were measured in the spalled regions of the surface where emergent grain boundaries of the substrate were present. Bending tests in the Auger device at room temperature confirmed this result: in the oxide areas cracked by bending, sulfur and carbon were found at the emergence of grain boundaries of the metal but neither sulfur nor carbon were observed in the cracked zones away from the grain boundaries of the metal. Yttrium was measured at the metal±oxide interface and in the grain boundaries of the oxide. No sulfur was found in the grain boundaries of the oxide. When yttrium was found at the metal±oxide interface, no sulfur could be detected. Chromium enrichment was observed at the metal±oxide interface in parallel with sulfur segregation but in yttrium containing alloys Cr enrichment disappeared at the same time as sulfur.
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